In recent years, in association with a reduction in size of an electrical or electronic component and an increase in power thereof, it has become a problem how heat generated from the electronic component or the like is radiated in a narrow space. An insulating adhesive or sheet that conducts heat from a portion of the electronic component from which the heat is to be generated to a heat-radiating member has been used as one means for the radiation. A composition obtained by filling a thermosetting resin with an inorganic high heat-radiating filler has been used in such adhesive or sheet. However, the quantity of heat generated from electronic equipment or an electronic component tends to increase, and hence an additional improvement in heat conductivity of the adhesive or sheet to be used for such equipment or component has been required. To this end, the resin needs to be filled with a higher concentration of the inorganic high heat-radiating filler. When the composition is used as the insulating adhesive or sheet, its heat conductivity in its thickness direction needs to be increased. Spherical alumina (having a heat conductivity of 36 W/m·K) or crystalline silica (12 W/m·K) is available as a filler suitable for the need. However, an improvement in heat radiation property achieved with the heat conductivity of such filler is limited. In view of the foregoing, in recent years, an agglomerated particle of hexagonal boron nitride (hereinafter sometimes abbreviated as “hBN”) has been attracting attention as a filler that improves heat radiation property in a thickness direction. The crystal structure of each of primary particles of hBN is a graphite-like hexagonal network layer structure. In addition, the particle has a scaly shape and has the following characteristic. The heat conductivity of the scaly-shaped particle in its surface direction is about 20 times (60 to 80 W/m·K) as large as its heat conductivity in its thickness direction, in other words, the heat conductivities are anisotropic. The hBN agglomerated particle is obtained by the agglomeration of the primary particles and has a characteristic in that its heat conductivities are isotropic, and hence the heat radiation property of a molded body containing the particle in its thickness direction can be significantly improved.
As related art documents disclosing heat radiation compositions using hBN agglomerated particles, there are known JP 2003-60134 A (U.S. Pat. No. 6,831,031) (Patent Document 1), JP 2009-24126 A (Patent Document 2), and JP 2011-6586 A (Patent Document 3).
In Patent Document 1, an hBN primary particle and the hBN agglomerated particle have been used in combination, but a heat conductivity in a thickness direction of only up to 3.8 W/m·K has been obtained.
In addition, Patent Document 2 makes a proposal to the effect that when an hBN agglomerated particle having a hardness based on a constant nanoindentation method of 500 MPa or more is used, the hBN agglomerated particle does not break at the time of molding. However, a heat conductivity in a thickness direction of only up to 10 W/m·K has been obtained.
In addition, Patent Document 3 makes the following proposal. Two kinds of hBN agglomerated particles having different agglomeration strengths are used, and a heat conductivity in a thickness direction is increased by achieving closest packing through the deformation and collapse of the hBN agglomerated particle having a lower agglomeration strength at the time of molding with a press. In the proposal, however, the press molding needs to be performed under such a condition that the hBN agglomerated particle having a higher agglomeration strength does not break, and hence pressure control may be difficult.